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<title>std::counted_iterator<I>::counted_iterator</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::counted_iterator<I>::</span>counted_iterator</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-iterator.html">iterator</a>‎ | <a href="cpp-iterator-counted_iterator.html">counted iterator</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> counted_iterator<span class="br0">(</span><span class="br0">)</span> requires std<span class="sy4">::</span><span class="me2">default_initializable</span><span class="sy1"><</span>I<span class="sy1">></span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> counted_iterator<span class="br0">(</span> I x, std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> n <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> I2 <span class="sy1">></span><br /></span>
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<p> requires std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span><span class="kw4">const</span> I2<span class="sy3">&</span>, I<span class="sy1">></span><br /></p>
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<span class="kw4">constexpr</span> counted_iterator<span class="br0">(</span> <span class="kw4">const</span> counted_iterator<span class="sy1"><</span>I2<span class="sy1">></span><span class="sy3">&</span> other <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<p>构造新的迭代器适配器。</p>
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<div class="t-li1"><span class="t-li">1)</span> 默认构造函数。<a href="cpp-language-value_initialization.html">值初始化</a>底层迭代器并以 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 初始化底层<i>长度</i>。在结果迭代器上的操作拥有有定义行为当且仅当值初始化的 <code>I</code> 上的对应操作亦拥有有定义行为。</div>
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<div class="t-li1"><span class="t-li">2)</span> 以 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>x<span class="br0">)</span></span></span> 初始化底层迭代器并以 <code>n</code> 初始化底层<i>长度</i>。若 <code>n</code> 为负则行为未定义。</div>
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<div class="t-li1"><span class="t-li">3)</span> 以 <code>other</code> 的对应组分初始化底层迭代器与<i>长度</i>。</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>x</td>
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<td>-</td>
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<td>要适配的迭代器</td>
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<tr class="t-par">
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<td>n</td>
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<td>-</td>
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<td>到末尾的距离</td>
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<tr class="t-par">
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<td>other</td>
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<td>-</td>
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<td>要转换的迭代器适配器</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <algorithm></span>
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<span class="co2">#include <initializer_list></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <iterator></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">static</span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> pi <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">3</span>, <span class="nu0">1</span>, <span class="nu0">4</span>, <span class="nu0">1</span>, <span class="nu0">5</span>, <span class="nu0">9</span>, <span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// (1) 默认构造函数：</span>
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<span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="sy1"><</span><a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">iterator</span><span class="sy1">></span> i1<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>i1 <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">default_sentinel</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>i1.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// (2) 分别初始化迭代器与长度：</span>
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<span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="sy1"><</span><a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">iterator</span><span class="sy1">></span> i2<span class="br0">{</span>
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pi.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, pi.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span> <span class="nu0">2</span>
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<span class="br0">}</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>i2.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">5</span><span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span><span class="sy2">*</span>i2 <span class="sy1">==</span> <span class="nu0">3</span> <span class="sy3">&&</span> i2<span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span> <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// (3) 转换构造函数：</span>
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std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="sy1"><</span><a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">iterator</span><span class="sy1">></span> i3<span class="br0">{</span>i2<span class="br0">}</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">copy</span><span class="br0">(</span>i3, std<span class="sy4">::</span><span class="me2">default_sentinel</span>,
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<a href="cpp-iterator-ostream_iterator.html"><span class="kw670">std::<span class="me2">ostream_iterator</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">int</span><span class="sy1">></span><span class="br0">{</span><a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>, <span class="st0">" "</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">3 1 4 1 5</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-counted_iterator-operator=.html"><span class="t-lines"><span>operator=</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>赋值另一迭代器适配器<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="visualClear"></div>
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